This invention relates to apparatus for applying a powdered coating to a workpiece. More particularly, this invention relates to apparatus for applying a heat-curable powdered coating to an electric motor component. Most particularly, this invention relates to apparatus for electrostatically applying an insulating coating to electric motor armatures and stators.
It is a common practice to insulate electric motor winding parts--i.e., armatures and stators--with a resinous or epoxy coating applied in the form of a powder. The motor part is either heated before powder application, so that the coating fuses on contact and flows into a more or less uniform layer, or the motor part is heated after powder application to fuse the coating.
One popular technique for assuring the desired regularity of powder application is to apply the powder using electrostatic methods. In these methods, the motor part is grounded and charged powder particles are deposited electrostatically. One electrostatic deposition method is to place the grounded motor part in a fluidized bed of powder particles suspended in a flow of ionized air. Another method uses electrostatic spray devices. A major advantage of electrostatic methods is that areas that need thicker than normal insulating coatings because they tend to develop more intense electric fields--e.g., corners--receive thicker coatings for the very same reason, as the more intense electric fields in those areas cause more particles to be deposited.
Fluidized bed coating is generally employed where the desired coating is relatively thick and relatively uniform, and where an even demarcation is desired between coated and uncoated areas. Fluidized bed coating is particularly useful where the object to be coated has reentrant portions that cannot be reached by direct spraying.
On the other hand, in cases where only isolated areas of the workpiece need to be coated, spray coating can be employed, even though it involves higher equipment costs and higher tolerance deviation in the finished coating. To the extent that both methods require recovery of unused powder, spray coating also has higher powder recovery costs. Spray coating can use electrostatic or non-electrostatic sprays.
In another known type of coating process, the workpiece is preheated before the powder is applied, so that it immediately fuses at least sufficiently to be retained on the workpiece.
In any type of coating process, before the actual coating step, the workpiece is cleaned to remove any dirt, grease, or other foreign matter such as solutions used on the workpiece to facilitate cutting or stamping. The workpiece is then masked to cover any areas that should not be coated. For example, in the case of an electric motor part, there may be areas where it will be necessary to make electrical contact for proper functioning of the motor or, more likely, there will be areas--such as the armature shaft--where the coating would result in too high a coefficient of friction, or where it would increase the dimensions of the coated piece to the point that it would no longer meet the necessary clearances--such as the rounded outer surface of the armature. If such areas are not masked, it may be possible to remove the powdered coating before the powder is fused by heating. However, in some cases the areas to be kept clean are difficult to reach with powder removal or cleaning devices, so it is easier to mask those areas to prevent powder deposition in the first place.
While automatic mask-applying devices are known, it may be difficult to apply masks automatically to certain areas. As a result, automatic mask-applying devices may be unnecessarily complex and therefore unnecessarily expensive.
Just as the workpiece must be treated before coating, it must also be treated after coating and before curing to remove, or clean, excess or undesired powder from areas where the workpiece should not be coated. Known cleaning techniques include use of vacuum, brushing, scraping, or wiping with open-cell foam material similar to sponge. These techniques require special handling of the workpiece to assure that the correct areas of the workpiece are presented to the cleaning devices. These techniques also require relative motion between the workpiece and the device, as well as powder recovery equipment to capture the removed powder (except in the case of vacuum cleaning applications which are in and of themselves recovery applications). Of course, if the preheating type of application technique is used, any powder applied cannot be removed, so no cleaning step is performed.
After cleaning, the workpiece is frequently heated sufficiently to set the coating to prevent its being dislodged in further handling, although this step may be omitted in some applications, and is always omitted where preheating was used. When such a "precuring" step is used, it is usually carried out by infrared, microwave or induction heating, requiring, in some cases, that the workpiece be placed in particular alignments for proper heating.
If precuring is not used, the masks cannot be removed prior to final curing without the possibility of dislodging powder from areas to be coated. However, after precuring, the masks may be removed and the workpiece heated to transform the coating into its desired final state. This final curing step can be carried out in different types of equipment, and sometimes includes a controlled cooling step after heating. Final curing occurs immediately after coating where the preheating technique is used, as the other intermediate steps are neither necessary nor possible.
Known apparatus for carrying out all of the steps of coating processes as described above is large, usually being made up of a number of separate units, requiring complex handling as the workpiece is removed from the production line for treatment at the various units. In addition, each unit has its own support equipment, such as, in particular, its own excess powder recovery system, requiring the moving of large volumes of air to recover excess powder.
It would be desirable to be able to provide apparatus for carrying out coating of workpieces that is relatively compact, with as few separate stations as possible.
It would also be desirable to be able to provide a common excess powder recovery system for the various units of the apparatus, to reduce the volume of air that must be handled.
It would further be desirable to be able to provide for simplified and efficient handling of the workpieces as they are removed from the production line, treated, and returned to the production line.
It would still further be desirable to be able to provide such handling apparatus that can position the workpieces relative to the various units of the apparatus.
It would yet further be desirable to be able to provide a more efficient cleaning unit for such apparatus.
Finally, it would be desirable to be able to provide simplified automatic masking and unmasking devices for such apparatus.